Turbofan gas turbine engines (which may be referred to simply as ‘turbofans’) are typically employed to power aircraft. Turbofans are particularly useful on commercial aircraft where fuel consumption is a primary concern. Typically a turbofan gas turbine engine will comprise an axial fan driven by an engine core. The engine core is generally made up of one or more turbines which drive respective compressors via coaxial shafts. The fan is usually driven directly off an additional lower pressure turbine in the engine core.
The fan comprises an array of radially extending fan blades mounted on a rotor and will usually provide, in current high bypass gas turbine engines, around seventy-five percent of the overall thrust generated by the gas turbine engine. The remaining portion of air from the fan is ingested by the engine core and is further compressed, combusted, accelerated and exhausted through a nozzle. The engine core exhaust mixes with the remaining portion of relatively high-volume, low-velocity air bypassing the engine core through a bypass duct.
The fan is radially surrounded by a fan casing. The fan casing is generally provided with a fan track liner. The fan track liner typically includes an annular layer of abradable material which surrounds the fan blades. During operation of the engine, the fan blades rotate freely within the fan track liner. At maximum speed the blades may cut a path into this abradable layer creating a seal against the fan casing and minimising air leakage around the blade tips during cruise. Further incursions can occur during gusts or take off rotations over time.
If a failure event occurred, a fan blade may be released from the remainder of the fan, and the fan casing and/or fan track liner are arranged so as to prevent the released blade from causing serious damage (that is, the fan casing and/or fan track liner are arranged to contain any high energy debris). The casing and fan track liner may do this in a variety of ways.
If the fan case is metallic, a hook may be provided to arrest forward movement of a released fan blade from the engine. If the fan blades are composite then the blades may break up on impact with the casing, which can ease containment of debris by distributing impact loads. A composite casing without the benefit of a containment hook may be designed to be longer in an axial direction than a metallic casing to cover the blade release trajectory. Such a casing may include features to assist in the break-up of composite fan blades.
Once a fan blade has been released from the remainder of the fan, the fan can become out of balance. An out of balance fan can cause vibrations that increase the risk of damage to the engine and an attached pylon structure and can also affect the pilotability of an aircraft powered by the gas turbine engine.
U.S. Pat. No. 8,647,049 proposes including one or more severance members arranged to trim the tips of the blades of an out of balanced fan. The severance members are provided radially between a gas washed surface of the fan track liner and the casing. When a fan is out of balance the orbit of the fan will change, and this can result in one or more of the blades of the fan being trimmed by the severance member.